Spin excitations under fields in an anisotropic bond-alternating quantum S=1 chain: contrast with Haldane spin chains (original) (raw)
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Magnetism: Molecules to Materials I, 2001
One-dimensional (1D) magnetic systems are very attractive because they exhibit magnetic properties which are rather different from those encountered at higher dimensionality, D = 2 or 3 . In 1D antiferromagnets (AF), in particular, the role of quantum magnetic fluctuations is expected to be very important. Quite generally, in ferromagnets (F), the quantum effects are strongly reduced by the increase of the correlation length as the temperature decreases, since a block of N ferromagnetically coupled spins is likely to behave as a single large spin N S. In AF, with isotropic short-range interactions, quantum effects are sizable even for D = 3. Indeed in Heisenberg AF with nearest neighbor interactions, the spontaneous magnetization has a zero point relative reduction of about 1/zS, where z is the number of nearest neighbors and S the spin value. At D = 1, the spin fluctuations have more drastic effects which were first pointed out by F.D.M. Haldane . Haldane conjectured that the 1D Heisenberg antiferromagnet (1D-HAF) with integer spin, in contrast to the case of half-integer spin, has a singlet ground state separated from the excited states by an energy gap, . The spatial correlation function of the 1D-HAF with halfinteger spin has a power law decay, while that for integer spin shows an exponential decay. Although fairly controversial at first, the Haldane conjecture was rapidly supported by numerical simulations on finite AF chains and by experiments on quasi-1D antiferromagnets . Such findings renewed the interest in 1D magnetism and induced a large amount of theoretical and experimental work, which is reviewed here.
Critical properties of coupled anisotropic Haldane spin chains in a magnetic field
Physical Review B
We use inelastic neutron scattering to measure magnetic fluctuations as a function of transverse magnetic field and temperature in a single crystal of the coupled Haldane chain compound SrNi2V2O8 with uniaxial anisotropy. At the base temperature (2 K), spin excitations confirm a field-induced quantum phase transition from the disordered Haldane (gapped singlet) state to a gapped long-range ordered state through a quantum critical point (QCP) (μ0H c ≈ 11.5 T). At elevated temperatures in the vicinity of the QCP, both the energy and linewidth of the lowest triplet mode scale linearly with temperature, in accordance with quantum criticality. We also demonstrate that the experimental field dependence of the triplet excitation modes at the base temperature is in agreement with the bosonic and macroscopic models, but is in contrast to the fermionic and perturbative models.
Physical Review B, 2008
The ground-state properties of a spin S = 1/2 tetrameric Heisenberg antiferromagnetic chain with alternating couplings AF1-AF2-AF1-F (AF and F denote antiferromagnetic and ferromagnetic couplings, respectively) are studied by means of the density matrix renormalization group method. Two plateaux of magnetization m are found at m = 0 and 1/4. It is shown that in such a spin-1/2 AF system, there is a gap from the singlet ground state to the triplet excited states in the absence of a magnetic field. The spin-spin correlation function decays exponentially, and the gapped states have a nonvanishing string order, which measures a hidden symmetry in the system. By a dual transformation, the string order is transformed into a ferromagnetic order and the hidden symmetry is unveiled to be a Z2 × Z2 discrete symmetry, which is fully broken in the gapped states. This half-integer spin Heisenberg AF chain is in a Haldane-like phase, suggesting that the present findings extend the substance of Haldane's scenario. A valence-bond-solid state picture is also proposed for the gapped states.
Magnetized States of Quantum Spin Chains
Lecture Notes in Physics, 2001
Quantum spin chains display complex cooperative phenomena that can be explored in considerable detail through theory, numerical simulations, and experiments. Here we review neutron scattering experiments that probe quantum spin chains in high magnetic fields. Experiments on copper-containing organometallic systems show that the uniform antiferromagnetic spin-l/2 chain has a gapless continuum of magnetic excitations and is critical in zero field. Application of a magnetic field creates incommensurate soft modes with a characteristic wave-vector that grows in proportion to the magnetization. These experimental results are evidence that the spins-l/2 chain maps to a one dimensional Luttinger Iiquid. Experiments on antiferromagnetic spin chains built from spin-l nickel atoms show a Haldane gap to bound triplet excitations and a finite critical field that must be exceeded to induce magnetization at low temperatures. These results indicate that the integer spin chain has an isolated singlet ground state with hidden topological order. For both spin-l/2 and spin-l systems, site alternation leads to a field induced gap in the excitation spectrum.
Progress of Theoretical Physics Supplement, 2005
We report the results of inelastic neutron scattering (INS) experiments on the S = 1 bond-alternating quasi-one-dimensional (1D) antiferromagnet Ni(C9D24N4)(NO2)ClO4 under magnetic fields below and above the critical field Hc at which the energy gap closes. Normal field dependence of Zeeman split of the excited triplet state below Hc has been observed, but the highest mode is unusually small and smears out with increasing field. This is probably explained by the highest branch buried in a magnon continuum, which was never observed in the S = 1 antiferromagnetic uniform chain. Above Hc, we observed only one excitation branch, which is completely different from the Haldane material NDMAP [A. Zheludev et al., Phys. Rev. B 68 (2003), 134438] where three gapped branches were clearly observed. §1. Introduction
Excitations from a Bose-Einstein condensate of magnons in coupled spin ladders
Physical Review Letters, 2007
The weakly coupled quasi-one-dimensional spin ladder compound CH 3 2 CHNH 3 CuCl 3 is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in reference to spin-dimer materials. However, for the upper two massive magnon branches, the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point.
Physical Review B, 2019
The mixed spin-1 and spin-1/2 Heisenberg octahedral chain with regularly alternating monomeric spin-1 sites and square-plaquette spin-1/2 sites is investigated using variational technique, localizedmagnon approach, exact diagonalization (ED) and density-matrix renormalization group (DMRG) method. The investigated model has in a magnetic field an extraordinarily rich ground-state phase diagram, which includes the uniform and cluster-based Haldane phases, two ferrimagnetic phases of Lieb-Mattis type, two quantum spin liquids and two bound magnon crystals in addition to the fully polarized ferromagnetic phase. The lowest-energy eigenstates in a highly-frustrated parameter region belong to flat bands and hence, low-temperature thermodynamics above the bound magnoncrystal ground states can be satisfactorily described within the localized-magnon approach. The variational method provides an exact evidence for the magnon-crystal phase with a character of the monomer-tetramer ground state at zero field, while another magnon-crystal phase with a single bound magnon at each square plaquette is found in a high-field region. A diversity of quantum ground states gives rise to manifold zero-temperature magnetization curves, which may involve up to four wide intermediate plateaus at zero, one-sixth, one-third and two-thirds of the saturation magnetization, two quantum spin-liquid regions and two tiny plateaus at one-ninth and one-twelfth of the saturation magnetization corresponding to the fragmentized cluster-based Haldane phases.
Journal of Magnetism and Magnetic Materials, 2007
We report on the results obtained from studying electron spin resonance, magnetic susceptibility, specific heat and thermal expansion experiments on a metalorganic spin-dimer system, C 36 H 48 Cu 2 F 6 N 8 O 12 S 2 (TK91). According to the first principle Density Functional Theory calculations, the compound represents a 3D-coupled dimer system with intradimer coupling J 1 =k B $ 10 K and interdimer couplings J 2 =k B J3=kBJ 3 =k B J3=kB 1 K. The measurements have been performed on both pressed powder and single-crystal samples in external magnetic fields up to 12 T and at low temperatures down to $ 0.2 K. Susceptibility measurements reveal a spin-gap behavior consistent with the theoretical results. Furthermore, clear indications of a field-induced phase transition have been observed. A similar field-induced phase transition was also detected in an inorganic compound TlCuCl 3 and was interpreted as Bose-Einstein condensation (BEC) of magnons. The possibility of changing both the intradimer and interdimer couplings in TK91 by chemical substitutions makes the system a potentially good system to study BEC of magnons. r